fix(kobo): stop endless sync retries and cut round trips

Report success when Kobo rejects a delete for a tag it no longer has, so the device stops retrying
one it can never complete. Skip the full-library reconcile while a device is still draining pages,
and page fifty books at a time so a large re-announce costs a few round trips instead of hundreds.

Ref #1005

server/src/modules/kobo/kobo-sync.controller.test.ts

@@ -1,4 +1,4 @@
-import { HttpStatus } from '@nestjs/common';
+import { BadRequestException, HttpStatus } from '@nestjs/common';
 
 import { KoboSyncController } from './kobo-sync.controller';
 
@@ -26,6 +26,7 @@ describe('KoboSyncController', () => {
   const proxyService = {
     forward: vi.fn(),
     request: vi.fn(),
+    sendUpstream: vi.fn(),
   };
   const bookIdentityService = {
     resolveBookIdByEntitlementId: vi.fn(),
@@ -427,16 +428,69 @@ describe('KoboSyncController', () => {
     expect(proxyService.forward).not.toHaveBeenCalled();
   });
 
-  it('proxies delete-items requests for non-BookOrbit Kobo tags', async () => {
+  it('relays the upstream answer for delete-items requests on non-BookOrbit Kobo tags', async () => {
     const req = { method: 'POST', url: '/api/v1/kobo/token/v1/library/tags/kobo-tag/items/delete' };
     const reply = makeReply();
+    const upstream = { status: HttpStatus.OK, headers: {}, body: Buffer.from('[]') };
+    proxyService.request.mockResolvedValue(upstream);
 
-    await controller.deleteTagItems('kobo-tag', { deviceToken: 'token-1' } as never, req as never, reply as never);
+    await controller.deleteTagItems('kobo-tag', { deviceToken: 'token-1', deviceId: 7 } as never, req as never, reply as never);
 
-    expect(proxyService.forward).toHaveBeenCalledWith(req, reply, 'token-1');
+    expect(proxyService.request).toHaveBeenCalledWith(req, 'token-1');
+    expect(proxyService.sendUpstream).toHaveBeenCalledWith(reply, upstream);
     expect(reply.send).not.toHaveBeenCalled();
   });
 
+  it.each([
+    ['delete-items', HttpStatus.BAD_REQUEST],
+    ['delete-items', HttpStatus.NOT_FOUND],
+    ['delete-items', HttpStatus.GONE],
+  ])('reports %s success when Kobo answers %s for a tag it does not know', async (_label, status) => {
+    const req = { method: 'POST', url: '/api/v1/kobo/token/v1/library/tags/BL-MS-2/items/delete' };
+    const reply = makeReply();
+    proxyService.request.mockResolvedValue({ status, headers: {}, body: Buffer.from('') });
+
+    await controller.deleteTagItems('BL-MS-2', { deviceToken: 'token-1', deviceId: 7 } as never, req as never, reply as never);
+
+    expect(reply.status).toHaveBeenCalledWith(HttpStatus.OK);
+    expect(reply.send).toHaveBeenCalledWith({ RequestResult: 'Success' });
+    expect(proxyService.sendUpstream).not.toHaveBeenCalled();
+  });
+
+  it('relays upstream failures that do not mean the tag is already gone', async () => {
+    const req = { method: 'POST', url: '/api/v1/kobo/token/v1/library/tags/BL-MS-2/items/delete' };
+    const reply = makeReply();
+    const upstream = { status: HttpStatus.UNAUTHORIZED, headers: {}, body: Buffer.from('') };
+    proxyService.request.mockResolvedValue(upstream);
+
+    await controller.deleteTagItems('BL-MS-2', { deviceToken: 'token-1', deviceId: 7 } as never, req as never, reply as never);
+
+    expect(proxyService.sendUpstream).toHaveBeenCalledWith(reply, upstream);
+    expect(reply.send).not.toHaveBeenCalledWith({ RequestResult: 'Success' });
+  });
+
+  it('lets a rejected proxy path stay a client error instead of an upstream fault', async () => {
+    const req = { method: 'DELETE', url: '/api/v1/kobo/token/v1/library/tags/kobo-tag' };
+    const reply = makeReply();
+    proxyService.request.mockRejectedValue(new BadRequestException('Invalid proxy path'));
+
+    await expect(controller.deleteTag('kobo-tag', { deviceToken: 'token-1', deviceId: 7 } as never, req as never, reply as never)).rejects.toThrow(
+      BadRequestException,
+    );
+    expect(reply.send).not.toHaveBeenCalled();
+  });
+
+  it('answers 502 when the upstream tag delete cannot be reached', async () => {
+    const req = { method: 'DELETE', url: '/api/v1/kobo/token/v1/library/tags/kobo-tag' };
+    const reply = makeReply();
+    proxyService.request.mockRejectedValue(new Error('socket hang up'));
+
+    await controller.deleteTag('kobo-tag', { deviceToken: 'token-1', deviceId: 7 } as never, req as never, reply as never);
+
+    expect(reply.status).toHaveBeenCalledWith(HttpStatus.BAD_GATEWAY);
+    expect(reply.send).not.toHaveBeenCalledWith({ RequestResult: 'Success' });
+  });
+
   it('acknowledges add-items requests for synced BookOrbit collection tags', async () => {
     const req = { method: 'POST', url: '/api/v1/kobo/token/v1/library/tags/col-1/items' };
     const reply = makeReply();
@@ -469,11 +523,23 @@ describe('KoboSyncController', () => {
     expect(proxyService.forward).not.toHaveBeenCalled();
   });
 
-  it('proxies delete-tag requests for non-BookOrbit Kobo tags', async () => {
+  it('reports success when Kobo no longer has the tag the device is deleting', async () => {
-    const req = { method: 'DELETE', url: '/api/v1/kobo/token/v1/library/tags/kobo-tag' };
+    const req = { method: 'DELETE', url: '/api/v1/kobo/token/v1/library/tags/a413bef9' };
+    const reply = makeReply();
+    proxyService.request.mockResolvedValue({ status: HttpStatus.NOT_FOUND, headers: {}, body: Buffer.from('') });
+
+    await controller.deleteTag('a413bef9', { deviceToken: 'token-1', deviceId: 7 } as never, req as never, reply as never);
+
+    expect(reply.status).toHaveBeenCalledWith(HttpStatus.OK);
+    expect(reply.send).toHaveBeenCalledWith({ RequestResult: 'Success' });
+    expect(proxyService.sendUpstream).not.toHaveBeenCalled();
+  });
+
+  it('never turns a rejected add-items request into a success', async () => {
+    const req = { method: 'POST', url: '/api/v1/kobo/token/v1/library/tags/BL-MS-2/items' };
     const reply = makeReply();
 
-    await controller.deleteTag('kobo-tag', { deviceToken: 'token-1' } as never, req as never, reply as never);
+    await controller.addTagItems('BL-MS-2', { deviceToken: 'token-1', deviceId: 7 } as never, req as never, reply as never);
 
     expect(proxyService.forward).toHaveBeenCalledWith(req, reply, 'token-1');
     expect(reply.send).not.toHaveBeenCalled();

server/src/modules/kobo/kobo-sync.controller.ts

@@ -5,6 +5,7 @@ import {
   Get,
   Header,
   HttpCode,
+  HttpException,
   HttpStatus,
   Logger,
   Param,
@@ -28,6 +29,7 @@ import { KoboTokenGuard } from './guards/kobo-token.guard';
 import { KoboSettingsService } from './services/kobo-settings.service';
 import { KoboSyncService } from './services/kobo-sync.service';
 import { KoboReadingStateService } from './services/kobo-reading-state.service';
+import type { KoboProxyResponse } from './services/kobo-proxy.service';
 import { KoboProxyService } from './services/kobo-proxy.service';
 import { KOBO_STORE_RESOURCES } from './kobo-store-resources';
 import { KoboBookIdentityService } from './services/kobo-book-identity.service';
@@ -37,6 +39,12 @@ import { decodeSyncToken, isUsableKoboSyncToken, withKoboSyncToken } from './ser
 const STORE_SYNC_TIMEOUT_MS = 8_000;
 const KOBO_SYNC_TOKEN_HEADER = 'x-kobo-synctoken';
 
+// Kobo answers a delete for a tag it does not know with a client error, and the device keeps the
+// operation queued until it succeeds, so relaying that error strands a tag that no longer exists
+// anywhere in a delete it can never complete. Server faults and auth failures still reach the
+// device, because those mean "try again later" rather than "already gone".
+const TAG_ALREADY_GONE_STATUSES: number[] = [HttpStatus.BAD_REQUEST, HttpStatus.NOT_FOUND, HttpStatus.GONE];
+
 type StoreSyncResult = {
   entitlements: unknown[];
   hasMore: boolean;
@@ -278,7 +286,7 @@ export class KoboSyncController {
     @Req() req: FastifyRequest,
     @Res() reply: FastifyReply,
   ) {
-    if (!isBookOrbitTag(tagId)) return this.proxyService.forward(req, reply, device.deviceToken);
+    if (!isBookOrbitTag(tagId)) return this.forwardTagDelete(req, reply, device, tagId);
     reply.status(HttpStatus.OK).send({ RequestResult: 'Success' });
   }
 
@@ -292,10 +300,43 @@ export class KoboSyncController {
   @Delete('v1/library/tags/:tagId')
   @HttpCode(HttpStatus.OK)
   async deleteTag(@Param('tagId') tagId: string, @KoboDevice() device: KoboDeviceContext, @Req() req: FastifyRequest, @Res() reply: FastifyReply) {
-    if (!isBookOrbitTag(tagId)) return this.proxyService.forward(req, reply, device.deviceToken);
+    if (!isBookOrbitTag(tagId)) return this.forwardTagDelete(req, reply, device, tagId);
     reply.status(HttpStatus.OK).send({ RequestResult: 'Success' });
   }
 
+  /**
+   * Forwards a delete for a tag BookOrbit does not own, reporting success when Kobo says the tag is
+   * not there. Only deletes get this treatment: a failed add must stay a failure, because reporting
+   * success would drop the device's pending change instead of retrying it.
+   */
+  private async forwardTagDelete(req: FastifyRequest, reply: FastifyReply, device: KoboDeviceContext, tagId: string): Promise<void> {
+    let response: KoboProxyResponse;
+    try {
+      response = await this.proxyService.request(req, device.deviceToken);
+    } catch (error: unknown) {
+      // A path that does not resolve to Kobo is rejected before the request leaves, and that stays
+      // the client error the shared proxy raises rather than becoming an upstream fault.
+      if (error instanceof HttpException) throw error;
+      const errorClass = error instanceof Error ? error.name : 'UnknownError';
+      const errorMessage = sanitizeLogValue(error instanceof Error ? error.message : 'unknown error');
+      this.logger.warn(
+        `[kobo.tag_delete] [fail] deviceId=${device.deviceId} errorClass=${errorClass} error="${errorMessage}" - upstream tag delete failed`,
+      );
+      reply.status(HttpStatus.BAD_GATEWAY).send({ message: 'Upstream Kobo API unavailable' });
+      return;
+    }
+
+    if (TAG_ALREADY_GONE_STATUSES.includes(response.status)) {
+      this.logger.warn(
+        `[kobo.tag_delete] [end] deviceId=${device.deviceId} tagId="${sanitizeLogValue(tagId)}" status=${response.status} - upstream does not know this tag, reporting success so the device stops retrying`,
+      );
+      reply.status(HttpStatus.OK).send({ RequestResult: 'Success' });
+      return;
+    }
+
+    this.proxyService.sendUpstream(reply, response);
+  }
+
   @Get('v1/library/:bookId/metadata')
   async getBookMetadata(
     @Param('bookId') bookId: string,

server/src/modules/kobo/services/kobo-proxy.service.ts

@@ -76,19 +76,22 @@ export class KoboProxyService {
     return { status: upstream.status, headers: responseHeaders, body: Buffer.from(await upstream.arrayBuffer()) };
   }
 
+  /** Relays an upstream response to the device, dropping the headers that must not be forwarded. */
+  sendUpstream(reply: FastifyReply, response: KoboProxyResponse): void {
+    reply.status(response.status);
+    for (const [key, value] of Object.entries(response.headers)) {
+      if (!HOP_BY_HOP_HEADERS.includes(key)) {
+        reply.header(key, value);
+      }
+    }
+    reply.send(response.body);
+  }
+
   async forward(req: FastifyRequest, reply: FastifyReply, deviceToken: string) {
     const targetUrl = this.resolveTargetUrl(req, deviceToken);
 
     try {
-      const { status, headers, body } = await this.request(req, deviceToken);
+      this.sendUpstream(reply, await this.request(req, deviceToken));
-
-      reply.status(status);
-      for (const [key, value] of Object.entries(headers)) {
-        if (!HOP_BY_HOP_HEADERS.includes(key)) {
-          reply.header(key, value);
-        }
-      }
-      reply.send(body);
     } catch (err) {
       this.logger.warn(`Proxy failed for ${targetUrl}: ${(err as Error).message}`);
       reply.status(502).send({ message: 'Upstream Kobo API unavailable' });

server/src/modules/kobo/services/kobo-sync.service.test.ts

@@ -238,8 +238,8 @@ describe('KoboSyncService', () => {
     expect(createSpy).toHaveBeenNthCalledWith(2, 5, 202, eligible, new Set());
     expect((service as any).fetchEligibleSnapshotRows).toHaveBeenNthCalledWith(1, 5, false, expect.any(Map));
     expect((service as any).fetchEligibleSnapshotRows).toHaveBeenNthCalledWith(2, 5, true, expect.any(Map));
-    expect(pageSpy).toHaveBeenNthCalledWith(1, 5, 11, 'device-a-token', 'https://reader.example.com', new Set([1]), expect.any(Map));
+    expect(pageSpy).toHaveBeenNthCalledWith(1, 5, 11, 'device-a-token', 'https://reader.example.com', expect.any(Map), expect.any(Function));
-    expect(pageSpy).toHaveBeenNthCalledWith(2, 5, 22, 'device-b-token', 'https://reader.example.com', new Set([1]), expect.any(Map));
+    expect(pageSpy).toHaveBeenNthCalledWith(2, 5, 22, 'device-b-token', 'https://reader.example.com', expect.any(Map), expect.any(Function));
   });
 
   it('reconciles only the caller device snapshot when it already exists', async () => {
@@ -254,7 +254,38 @@ describe('KoboSyncService', () => {
     await service.getDelta(5, 101, 'device-a-token', 'https://reader.example.com');
 
     expect(reconcileSpy).toHaveBeenCalledWith(9, eligible);
-    expect(pageSpy).toHaveBeenCalledWith(5, 9, 'device-a-token', 'https://reader.example.com', new Set([1]), expect.any(Map));
+    expect(pageSpy).toHaveBeenCalledWith(5, 9, 'device-a-token', 'https://reader.example.com', expect.any(Map), expect.any(Function));
+    await expect(pageSpy.mock.calls[0]![5]()).resolves.toEqual(new Set([1]));
+  });
+
+  it('serves the next page without reconciling while the device still has pending rows', async () => {
+    const db = makeDb({ select: [[{ bookId: 4 }]] });
+    const service = makeService(db);
+    vi.spyOn(service as any, 'findDeviceSnapshot').mockResolvedValue({ id: 9, userId: 5, deviceId: 101 });
+    const eligibleSpy = vi.spyOn(service as any, 'fetchEligibleSnapshotRows').mockResolvedValue([]);
+    const reconcileSpy = vi.spyOn(service as any, 'reconcileSnapshot').mockResolvedValue(undefined);
+    const pageSpy = vi.spyOn(service as any, 'getPageFromSnapshot').mockResolvedValue({ entitlements: [], hasMore: true, syncToken: 'PX.token' });
+
+    await service.getDelta(5, 101, 'device-a-token', 'https://reader.example.com');
+
+    expect(reconcileSpy).not.toHaveBeenCalled();
+    expect(eligibleSpy).not.toHaveBeenCalled();
+    expect(pageSpy).toHaveBeenCalledWith(5, 9, 'device-a-token', 'https://reader.example.com', expect.any(Map), expect.any(Function));
+  });
+
+  it('still resolves eligible ids for tag delivery when a pending page drains before it is served', async () => {
+    const db = makeDb({ select: [[{ bookId: 4 }]] });
+    const service = makeService(db);
+    vi.spyOn(service as any, 'findDeviceSnapshot').mockResolvedValue({ id: 9, userId: 5, deviceId: 101 });
+    const eligibleSpy = vi
+      .spyOn(service as any, 'fetchEligibleSnapshotRows')
+      .mockResolvedValue([{ bookId: 7, fileHash: 'h', deliveryHash: 'd', metadataHash: 'm', needsLegacyNumericRemoval: false }]);
+    const pageSpy = vi.spyOn(service as any, 'getPageFromSnapshot').mockResolvedValue({ entitlements: [], hasMore: false, syncToken: 'PX.token' });
+
+    await service.getDelta(5, 101, 'device-a-token', 'https://reader.example.com');
+
+    await expect(pageSpy.mock.calls[0]![5]()).resolves.toEqual(new Set([7]));
+    expect(eligibleSpy).toHaveBeenCalledWith(5, true, expect.any(Map));
   });
 
   it('getBookMetadata returns empty array when book is not eligible', async () => {
@@ -464,7 +495,7 @@ describe('KoboSyncService', () => {
     const service = makeService(db);
     vi.spyOn(service as any, 'buildTagItems').mockResolvedValue([{ ChangedTag: {} }]);
 
-    const result = await (service as any).getPageFromSnapshot(7, 1, 'tok', 'https://base', new Set([1]), new Map());
+    const result = await (service as any).getPageFromSnapshot(7, 1, 'tok', 'https://base', new Map(), () => Promise.resolve(new Set([1])));
 
     expect(result).toEqual({
       entitlements: [{ ChangedTag: {} }],
@@ -491,7 +522,7 @@ describe('KoboSyncService', () => {
     );
     readingStateService.getRawState.mockResolvedValue(null);
 
-    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Set([1, 2, 3]), new Map());
+    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Map(), () => Promise.resolve(new Set([1, 2, 3])));
 
     expect(result.hasMore).toBe(false);
     expect(result.entitlements).toHaveLength(3);
@@ -510,7 +541,7 @@ describe('KoboSyncService', () => {
     vi.spyOn(service as any, 'fetchEligibleBooksByIds').mockResolvedValue(new Map([[3, makeBook(3)]]));
     readingStateService.getRawState.mockResolvedValue({ EntitlementId: 'entitlement-3', CurrentBookmark: { ProgressPercent: 61 } });
 
-    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Set([3]), new Map());
+    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Map(), () => Promise.resolve(new Set([3])));
 
     expect(result.entitlements).toHaveLength(2);
     expect(result.entitlements[0]).toHaveProperty('ChangedProductMetadata');
@@ -530,7 +561,7 @@ describe('KoboSyncService', () => {
     bookIdentityService.findByBookIds.mockResolvedValue(new Map([[3, makeIdentity(3, false)]]));
     readingStateService.getRawState.mockResolvedValue({ EntitlementId: 'entitlement-3', CurrentBookmark: { ProgressPercent: 61 } });
 
-    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Set([3]), new Map());
+    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Map(), () => Promise.resolve(new Set([3])));
 
     expect(result.entitlements).toHaveLength(2);
     expect(result.entitlements[0]).toHaveProperty('ChangedEntitlement');
@@ -550,7 +581,7 @@ describe('KoboSyncService', () => {
     vi.spyOn(service as any, 'fetchEligibleBooksByIds').mockResolvedValue(new Map());
     bookIdentityService.findByBookIds.mockResolvedValue(new Map([[3, makeIdentity(3, true)]]));
 
-    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Set(), new Map());
+    const result = await (service as any).getPageFromSnapshot(7, 22, 'tok', 'https://base', new Map(), () => Promise.resolve(new Set()));
     const removedIds = result.entitlements.map(
       (entry: Record<string, { BookEntitlement: { Id: string } }>) => entry.ChangedEntitlement.BookEntitlement.Id,
     );

server/src/modules/kobo/services/kobo-sync.service.ts

@@ -18,7 +18,10 @@ import { encodeSyncToken } from './kobo-sync-token';
 
 type Db = NodePgDatabase<typeof schema>;
 
-const SYNC_PAGE_SIZE = 5;
+// A device pauses between sync rounds, so wall-clock time for a long delta tracks the number of
+// round trips rather than the work inside each one: at five books a page, re-announcing a few
+// hundred books cost users tens of minutes of waiting on a mostly idle server.
+export const SYNC_PAGE_SIZE = 50;
 const SNAPSHOT_RECONCILE_BATCH_SIZE = 5000;
 const SNAPSHOT_CREATE_BATCH_SIZE = 5000;
 const METADATA_SERIALIZER_VERSION = 2;
@@ -67,6 +70,8 @@ type SmartScopeMatch = { name: string; bookIds: number[]; where: SQL | undefined
 // caching across requests (this service is a singleton).
 type SmartScopeMatchCache = Map<number, Promise<Map<number, SmartScopeMatch>>>;
 
+type EligibleIdsResolver = () => Promise<Set<number>>;
+
 export interface KoboBookEntry {
   bookId: number;
   koboEntitlementId: string;
@@ -111,9 +116,19 @@ export class KoboSyncService {
     baseUrl: string,
   ): Promise<{ entitlements: unknown[]; hasMore: boolean; syncToken: string }> {
     let snapshot = await this.findDeviceSnapshot(userId, deviceId);
+    const smartScopeMatchCache: SmartScopeMatchCache = new Map();
+
+    // Reconciling costs a pass over the whole library, so a device still working through its
+    // pending rows skips it: those rows already say what it is owed, and whatever changed while
+    // it paged is picked up by the reconcile that opens its next sync.
+    if (snapshot && (await this.hasPendingSnapshotBooks(snapshot.id))) {
+      return this.getPageFromSnapshot(userId, snapshot.id, deviceToken, baseUrl, smartScopeMatchCache, () =>
+        this.fetchEligibleBookIds(userId, true, smartScopeMatchCache),
+      );
+    }
+
     const hadDeviceSnapshot = snapshot ? true : await this.hasDeviceSnapshot(userId);
     const legacyNumericRemovalBookIds = snapshot ? new Set<number>() : await this.getLegacyNumericRemovalBookIds(userId, deviceId);
-    const smartScopeMatchCache: SmartScopeMatchCache = new Map();
     const eligibleSnapshotRows = await this.fetchEligibleSnapshotRows(userId, hadDeviceSnapshot, smartScopeMatchCache);
 
     if (!snapshot) {
@@ -128,14 +143,8 @@ export class KoboSyncService {
       await this.reconcileSnapshot(snapshot.id, eligibleSnapshotRows);
     }
 
-    return this.getPageFromSnapshot(
+    const eligibleIds = new Set(eligibleSnapshotRows.map((row) => row.bookId));
-      userId,
+    return this.getPageFromSnapshot(userId, snapshot.id, deviceToken, baseUrl, smartScopeMatchCache, () => Promise.resolve(eligibleIds));
-      snapshot.id,
-      deviceToken,
-      baseUrl,
-      new Set(eligibleSnapshotRows.map((row) => row.bookId)),
-      smartScopeMatchCache,
-    );
   }
 
   async getBookMetadata(userId: number, bookId: number, deviceToken: string, baseUrl: string): Promise<unknown[]> {
@@ -172,6 +181,24 @@ export class KoboSyncService {
     });
   }
 
+  private async hasPendingSnapshotBooks(snapshotId: number): Promise<boolean> {
+    const [row] = await this.db
+      .select({ bookId: schema.koboSnapshotBooks.bookId })
+      .from(schema.koboSnapshotBooks)
+      .where(and(eq(schema.koboSnapshotBooks.snapshotId, snapshotId), eq(schema.koboSnapshotBooks.synced, false)))
+      .limit(1);
+    return Boolean(row);
+  }
+
+  private async fetchEligibleBookIds(
+    userId: number,
+    needsLegacyNumericRemovalForNewMappings: boolean,
+    smartScopeMatchCache: SmartScopeMatchCache,
+  ): Promise<Set<number>> {
+    const rows = await this.fetchEligibleSnapshotRows(userId, needsLegacyNumericRemovalForNewMappings, smartScopeMatchCache);
+    return new Set(rows.map((row) => row.bookId));
+  }
+
   private async hasDeviceSnapshot(userId: number): Promise<boolean> {
     const snapshot = await this.db.query.koboLibrarySnapshots.findFirst({
       where: eq(schema.koboLibrarySnapshots.userId, userId),
@@ -475,13 +502,18 @@ export class KoboSyncService {
     });
   }
 
+  /**
+   * Serves the next page of a snapshot. Eligibility is resolved lazily because only the final page
+   * needs the full set, and reaching for it on every page is what makes a long delta O(library) per
+   * request.
+   */
   private async getPageFromSnapshot(
     userId: number,
     snapshotId: number,
     deviceToken: string,
     baseUrl: string,
-    eligibleIds: Set<number>,
     smartScopeMatchCache: SmartScopeMatchCache,
+    resolveEligibleIds: EligibleIdsResolver,
   ): Promise<{ entitlements: unknown[]; hasMore: boolean; syncToken: string }> {
     const syncToken = encodeSyncToken(snapshotId);
 
@@ -496,7 +528,7 @@ export class KoboSyncService {
     const page = pending.slice(0, SYNC_PAGE_SIZE);
 
     if (page.length === 0) {
-      const tagItems = await this.buildTagItems(userId, eligibleIds, smartScopeMatchCache);
+      const tagItems = await this.buildTagItems(userId, await resolveEligibleIds(), smartScopeMatchCache);
       return { entitlements: tagItems, hasMore: false, syncToken };
     }
 

server/test/kobo-multi-device-sync.e2e-spec.ts

@@ -6,6 +6,7 @@ import { and, eq, inArray } from 'drizzle-orm';
 import { Client } from 'pg';
 
 import * as schema from '../src/db/schema';
+import { SYNC_PAGE_SIZE } from '../src/modules/kobo/services/kobo-sync.service';
 import { waitForCondition } from './e2e/app-harness';
 import { createEpubFixture } from './e2e/reader-state-isolation/reader-state-isolation-fixture-builder';
 import {
@@ -25,6 +26,9 @@ type NewEntitlement = { BookEntitlement: { Id: string }; ReadingState: Record<st
 
 const MIGRATIONS_DIRECTORY = join(dirname(fileURLToPath(import.meta.url)), '../src/db/migrations');
 
+// One book past a full page, so the suite keeps exercising a page boundary whatever SYNC_PAGE_SIZE is.
+const LIBRARY_BOOK_COUNT = SYNC_PAGE_SIZE + 1;
+
 function escapeIdentifier(identifier: string): string {
   return `"${identifier.replace(/"/g, '""')}"`;
 }
@@ -240,7 +244,7 @@ describe('Kobo multi-device library sync (e2e)', { timeout: 180_000 }, () => {
     library = await createLibraryWithFolder(ctx, { name: `kobo-multi-device-${randomUUID()}` });
 
     const paths = await Promise.all(
-      Array.from({ length: 6 }, (_, index) =>
+      Array.from({ length: LIBRARY_BOOK_COUNT }, (_, index) =>
         createEpubFixture(library.folderPath, `kobo-multi-device-${index + 1}.epub`, {
           title: `Kobo Multi Device ${index + 1}`,
           uid: `urn:uuid:${randomUUID()}`,
@@ -291,7 +295,7 @@ describe('Kobo multi-device library sync (e2e)', { timeout: 180_000 }, () => {
     const first = await sync(echoDevice);
     const firstEntitlements = newEntitlements(first.entries);
     expect(first.hasMore).toBe(true);
-    expect(firstEntitlements).toHaveLength(5);
+    expect(firstEntitlements).toHaveLength(SYNC_PAGE_SIZE);
 
     for (const entitlement of firstEntitlements) {
       await putReadingState(echoDevice, entitlement.BookEntitlement.Id, entitlement.ReadingState);
@@ -323,8 +327,8 @@ describe('Kobo multi-device library sync (e2e)', { timeout: 180_000 }, () => {
     const bFirst = await sync(deviceB);
     expect(aFirst.hasMore).toBe(true);
     expect(bFirst.hasMore).toBe(true);
-    expect(entitlementIds(aFirst.entries)).toHaveLength(5);
+    expect(entitlementIds(aFirst.entries)).toHaveLength(SYNC_PAGE_SIZE);
-    expect(entitlementIds(bFirst.entries)).toHaveLength(5);
+    expect(entitlementIds(bFirst.entries)).toHaveLength(SYNC_PAGE_SIZE);
 
     const aSecond = await sync(deviceA, aFirst.syncToken);
     expect(aSecond.hasMore).toBe(false);
@@ -373,7 +377,7 @@ describe('Kobo multi-device library sync (e2e)', { timeout: 180_000 }, () => {
           snapshots.map((snapshot) => snapshot.id),
         ),
       );
-    expect(rows).toHaveLength(12);
+    expect(rows).toHaveLength(LIBRARY_BOOK_COUNT * 2);
     expect(rows.every((row) => row.synced)).toBe(true);
   });
 
@@ -482,7 +486,7 @@ describe('Kobo multi-device library sync (e2e)', { timeout: 180_000 }, () => {
             .map((snapshot) => snapshot.id),
         ),
       );
-    expect(aAndBRows).toHaveLength(12);
+    expect(aAndBRows).toHaveLength(LIBRARY_BOOK_COUNT * 2);
     expect(aAndBRows.every((row) => row.synced)).toBe(true);
 
     const [targetIdentity] = await ctx.db